An A/D (analog-to-digital) converter 100 is shown in FIG. 5(a) as an example of a system responsive to differential inputs. The A/D converter 100 converts differential analog inputs, i.e., a non-inverting phase signal (IN(P)) and inverting phase signal (IN(N)) into a digital output signal (OUT). In such conversion of differential inputs, it is necessary to match the DC levels of the external analog input signals IN(P) and IN(N) (VDCP, VDCN) to the DC reference level (Vref) within the A/D converter 100.
FIG. 5(b) shows an example of prior circuits used for solving the problem of DC offset, wherein the conventional differential input interface circuit 102 consists of level shift circuits 104 and 114, and buffer circuits 106 and 116. The DC levels of the OUT (P) and OUT (N) (VDCP and VDCN) are adjusted by level shifting circuits 104 and 114 to match the DC level within the A/D converter 100, even if the DC levels of the external inputs IN(P) and IN(N) fluctuate. In the event wherein the analog input is part of a single line structure, the DC level of the input signal needs only to be shifted to the DC level (Vref) at which a correct A/D converter operation must operate. In the case where the analog input belongs to a differential line structure, the DC levels of the input signals are required to coincide to each other and, in addition, to the DC level (Vref) within the A/D converter 100. In other words, when there are differences between the DC levels of the analog inputs IN (P), IN (N) (VDCP, VDCN) and the DC level (Vref) within the A/D converter 100, as shown in FIG. 6(a), the DC level (VDCP) of the non-inverting phase signal OUT (P) is required not only to match the DC level (VDCN) of the inverting phase signal OUT (N), as shown in FIG. 6(b), but also to the DC level (Vref) within the A/D converter, as illustrated in FIG. 6(c).
FIG. 5(c) illustrates another type of conventional differential input interface circuit 112, consisting of capacitors 108 and 118. This input interface circuit 112 cuts-off the DC levels of the differential analog inputs IN (P) and IN (N) by way of AC coupling, using capacitors 108 and 118 for this purpose. The removal of the DC level eliminates the adverse effect of the DC offset (i.e., level fluctuation) both external to the A/D converter and from the interface. However, in this case, it is required to create an appropriate DC level within the input interface circuit 112 to ensure a correct operation of the A/D converter 100. This must be done simply and easily. In this manner, in order to eliminate the adverse effect of the offset from outside the A/D converter and the interface, it is required that the DC level of the external inputs match the correct operation level within the A/D converter.